1. Field of the Invention
The present invention relates to an image formation apparatus such as a printer, a photocopier, a facsimile machine, and the like, and specifically to an image formation apparatus employing a method wherein plural toner images each formed on an image holding member first are transferred by respective primary transfer operations onto an intermediate transfer member, superposed one on another, and then are transferred by a single secondary transfer operation onto a recording medium.
2. Description of the Related Art
FIG. 5 illustrates a conventional image formation apparatus and technique wherein toner images are transferred using an intermediate transfer member.
The surface of a photosensitive drum 1 is evenly charged by a charging roller 2, and then subjected to laser irradiation according to image information by an exposure device 3, thereby forming an electrostatic latent image thereon. The electrostatic latent image is developed (image manifestation) as a toner image t by toner having a charge (charged toner) being electrostatically adhered thereto by a developing device 4.
The toner image t on the photosensitive drum 1 is transferred in a primary transfer operation onto an intermediate transfer belt 5 (intermediate transfer member) at a primary transfer position (primary transfer nip) T1 by a primary transfer roller 6. This intermediate transfer belt 5 is reeved over a driving roller 21, a tension roller 22, and a secondary transfer inner roller 23. The toner image t on the intermediate transfer belt 5 is electrostatically transferred in a secondary transfer operation onto a recording medium P at a secondary transfer position (secondary transfer nip) T2 by a secondary transfer outer roller 24. The recording medium is transported into the secondary transfer position (nip) at a predetermined timing coinciding with the toner image t.
The recording medium P upon which the toner image t is transferred then is transported toward a fixing device 9 in the direction indicated by the arrow K1, and heated and pressed by the fixing device 9, whereby the toner image t is fixed onto the surface of the recording medium P.
Residual toner on the photosensitive drum 1 following the primary transfer operation is removed by a cleaning device 7, and residual toner on the intermediate transfer belt 5 following the secondary transfer operation is removed by an intermediate transfer member cleaner 10.
In an image formation apparatus using the above-described intermediate transfer method, the back side (interior facing side) of the intermediate transfer belt 5 is charged with reverse polarity relative to the charge polarity of the toner, in order to hold the charged toner on the surface of the intermediate transfer belt 5. That is to say, as shown in FIG. 6, in order to hold the charged toner at a negative polarity, for example, a positive charge applied to the back side of the intermediate transfer belt 5 electrostatically affects/holds the charged toner to the front surface of the intermediate transfer belt 5.
If the amount of charge on the back side of the intermediate transfer belt 5 varies (increases or decreases), the toner on the front side surface of the intermediate transfer belt 5 is electrostatically disturbed, which can result in unsuitable (degraded) images.
In such a case, as shown in FIG. 7, when line drawings or the like are formed on the intermediate transfer belt 5 as the belt 5 is passed through the primary transfer portion T1, marked scattering of toner occurs in front and back of the original line in the direction of travel(es) as the belt 5 approaches and passes the driving roller 21, which is the first contact member after (downstream of) the primary transfer position. As shown in FIG. 7, scattering of toner upstream and downstream of the line drawing becomes worse both before and after passing over the driving roller 21. Such scattering causes a widening of the line drawing in the direction of travel of the belt 5.
As shown in FIG. 8, a tension roller 22 over which the intermediate transfer belt 5 is reeved allows the charge on the back side of the intermediate transfer belt 5 to be diffused, thereby causing scattering of the toner particles held on the surface of the intermediate transfer belt 5.
Specifically, the charged back side of the intermediate transfer belt 5 comes into contact with the tension roller 22, which is electrically grounded, whereby charge escapes to ground. If a substantial amount of the charge on the back side of the intermediate transfer belt 5 escapes to ground, the amount of charge on the back side of the intermediate transfer belt 5 is less than the total amount of charge of the toner particles electrostatically drawn to the intermediate transfer belt 5, such that the force drawing the toner particles thereto is reduced, and toner particles scatter due to electrostatic repellence between the toner particles, resulting in this phenomenon.
On the other hand, Japanese Patent Laid-Open No. 2000-298408 and U.S. Pat. No. 6,298,212 disclose structures and methods in which a contact member which the intermediate transfer belt first comes into contact with after passing through the primary transfer position (such as a tension roller disposed immediately downstream of the primary transfer position in the moving direction of the intermediate transfer belt) controls the charge on the back side of the intermediate transfer belt 5 so that toner particles held thereon do not scatter.
Specifically, the following methods and arrangements are disclosed.
(1) The first contact member is grounded through a resistor with a high value.
(2) A high resistive layer (insulating layer) is provided on the surface layer of the first contact member.
(3) The first contact member is not grounded, thereby preventing exchange (drain) of charges.
(4) Applying a bias having the same polarity as the primary transfer to the first contact member, thereby holding the charge on the back side of the intermediate transfer belt 5.
However, the above-described means also have the following shortcomings.
(1) In the event of the first contact member being grounded through a resistor with a high value, providing the resistor with a high value leads to increased costs, and may also complicate the configuration in comparison with arrangements grounded without the resistor having a high value.
(2) In the event of providing a high resistive layer (insulating layer) on the surface of the first contact member, the insulating layer may peel off or become worn after long-time use, such that scattering of toner cannot be prevented in a reliable and stable manner for long-time use.
(3) In the event of the first contact member being not grounded, when continuously forming a great number of images, such as dozens to several hundred of images, the first contact member becomes charged up to several kV, which could damage the electric system of the image formation apparatus due to discharge of accumulated charge.
(4) In the event of applying a bias having the same polarity as the primary transfer to the first contact member, the configuration and control of the image formation apparatus may become complicated.